The present invention relates to a single longitudinal mode semiconductor laser which may be used as a light source in optical fiber communication systems.
Optical fiber communication has been rapidly put into practice as the means were developed to effectively reduce signal attenuation in optical fibers and to prolong the life of the semiconductor. Experiments were started to build systems capable of super long distance transmission (more than 100 km) in super-low-loss transmission bands of less than 0.5 dB/km that is attainable in the wavelengths of 1.3 microns and 1.5 microns. Their application to long distance trunk lines in submarine communication systems and the like is being considered, as it is desirable to extend the interval distance between repeaters in such a system.
In such a long distance transmission system, chromatic dispersion becomes another critical issue, in addition to the transmission loss which occurs in optical fibers. Semiconductor lasers are generally utilized as the light source in optical fiber communication systems, but the components of a conventional structure which uses the cleavage facets of a crystal as a Fabry-Perot resonator do not necessarily provide single longitudinal mode oscillation. Especially at the time of high speed modulation, as the number of oscillation mode increases, the interval between repeaters in a high speed communication system (such as 400 Mb/s and 1.6 Gb/s systems) is limited mainly by the chromatic dispersion rather than the transmission loss. In order to realize a long-distance and yet high-speed transmission system, it is therefore desirable to employ a semiconductor laser which is capable of a single longitudinal mode of oscillation even during high speed modulation.
As a semiconductor laser of this type, there have been proposed a distributed Bragg reflector semiconductor laser and a distributed feedback laser which do not use a Fabry-Perot resonator, but have built-in gratings of a periodic structure. Components of several structures have been made on a trial basis and have reached the level where room-temperature CW oscillation is possible. In one of the proposals, the distributed-Bragg-refection waveguide is disposed close to the boundary of the air while the active layer is farther away (Y. Abe et al, "GaInAsP/InP Integrated Laser with Butt-Jointed Built-in Distributed-Bragg-Reflection Waveguide", Electronics Letters, Vol. 17, No. 25, pp. 945-947 Dec. 10, 1981). This would invite some coupling loss between the active layer and the waveguide layer because the field patterns are not in coincidence. The forming of the good distributed-Bragg-reflection waveguide is not easy since the corrugation must be formed in the lower step of the multilayered structure. In another proposal, while the distributed-Bragg-reflector is fully embedded in the semiconductor material, the positional registration between the active layer and waveguide layer is rather difficult, thus effecting the coupling loss (U.S. patent application Ser. No. 447,553, filed Dec. 7, 1982). These structures are not always practically satisfactory in respect of oscillation threshold, and basic performance such as optical output, reliability, reproducability in manufacture, etc. A need has been keenly felt to develop a novel structure using an improved epitaxial growth process and high quality grown crystal.